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| Titel |
Interannual variability and decadal trends in carbon exchange at the Harvard Forest EMS site |
| VerfasserIn |
J. W. Munger, S. C. Wofsy, P. R. Moorcroft, D. Medvigy |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250024370
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| Zusammenfassung |
| The Harvard Forest EMS site in a mixed deciduous forest in central Massachusetts has been
measuring carbon, water, and energy fluxes since 1992. Above-ground biomass, litter input,
and tree mortality have been measured since 1995. The forest at this site has consistently
been a net sink for carbon over the measurement period with annual uptake rates
of 1.0 to > 5.Mg-C ha-1y-1. Carbon uptake rates show a significant increasing
trend, despite the forest being 75- 110 years old. There were parallel increases in
midsummer photosynthetic capacity at high light level (21.5-31.5 μmole m-2s-1),
woody biomass (101-115 Mg-C ha-1from 1993-2005, mostly due to growth of
one species, red oak), and peak leaf area index (4.5-5.5 m2m-2from 1998–2005).
These long-term trends were interrupted in 1998 by sharp declines in photosynthetic
capacity, net ecosystem exchange (NEE) of CO2, and other parameters, followed by
recovery over the next 3 years. The dip in 1998 could not be directly attributed
to any one cause, though leaf expansion in the spring appeared to stall during a
period of unfavorable weather, and did not recover later in the summer. Annual
increment of above-ground woody biomass has followed the trend in NEE with 1 year
offset implying that spring wood growth is supplied by carbon fixed in the previous
year.
An empirical model of carbon fluxes based on mean temperature and light response
functions and observed phenology represents the hourly to seasonal patterns in carbon
fluxes but can not adequately account for interannual variability or the long-term
trends in carbon uptake. A structured ecosystem model (ED2) that represented
both canopy-scale physiology and long-term dynamics of tree growth, mortality,
and species composition was able to simulate interannual variability over decadal
intervals better than the empirical model based on mean responses could. These results
imply that direct effects of climate variability only partially account for interannual
variability in NEE. Other key factors appear to be indirect effects of climate forcing
on leaf biomass and canopy performance, and long term successional trends in
species composition and structure. Detection and attribution of the factors that control
long-term trends and interannual variability requires continued long-term data records. |
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